1. Field of the Invention
The present invention relates to a white light emitting device using a Light Emitting Diode (LED) and, more particularly, to a white light emitting device which can significantly expand a range of the peak wavelength of an applicable LED by using a combination of orange phosphor and green phosphor.
2. Description of the Related Art
Recently, a white Light Emitting Diode (LED) device is extensively used as a backlight of a liquid crystal display device in replacement of a conventional small-sized lamp or fluorescent lamp. Typically, a white LED device is realized by a combination of a blue LED and yellow phosphor. The phosphor is excited by the blue light emitted from the LED, and thereby emits yellow light. The color combination of the blue light and the yellow light is perceived as white light by an observer. In this case, YAG:Ce and TAG:Ce phosphors, which are garnet crystals of YAG and TAG activated by Ce3+, are typically used for the yellow phosphor. In addition, Eu2+-activated silicate phosphor is widely used for the yellow phosphor for the white LED device, including for example, phosphors having compositions expressed by formulae: (2−x−y)SrO.x(Ba,Ca)O.(1−a−b−c−d)SiO2.aP2O5 bAl2O3.cB2O3.dGeO2:yEu2+, where 0<x<1.6, 0.005<y<0.5, and 0<a, b, c, d<0.5 and (3−x)SrO.SiO2:xEu2+, where 0<x<1. These phosphors are excited by the blue light to emit yellow light with relatively high quantum efficiency.
Each of the aforementioned yellow phosphors has a peculiar yellow peak wavelength. Therefore, in order to obtain white light in a region of x=0.27 to 0.33, y=0.25 to 0.35 in the 1931 CIE chromaticity coordinates, which correspond to pure white, the wavelength of the blue LED usable with the yellow phosphor is extremely limited. In order to overcome such drawbacks, there have been suggested methods of substituting Y with Gd or Al with Ga in the YAG:Ce phosphor and substituting Tb with Gd or Al with Ga in the TAG:Ce phosphor, thereby obtaining a short peak wavelength or a long peak wavelength of the phosphor. Also, in the case of a silicate phosphor, Sr, Ca and Ba can be substituted with each other to induce the wavelength change of the silicate yellow phosphor.
However, the shift range of the wavelength by adjusting the composition as described above is limited. Moreover, as the wavelength change in the phosphor results in degraded light emission efficiency in a corresponding wavelength region, the possible shift range of the wavelength in practice is further limited. In addition, in the case of using the conventional Garnet-based or silicate-based yellow phosphors, the shift range of the yellow light emission wavelength is very small, thus limiting the light emission wavelength of the blue LED used together to realize white light. This in turn hinders realization of high quality white light.